Support gRPC channel options

[alertedsnake]

I also work with gRPC in Go and Python, and both have a way to use the channel options to do a number of things, in my specific case I'd like to set the user-agent header, which cannot be done via metadata.

In Python it looks like this:

connect = 'localhost:50041'
user_agent = "monkey 1.0.0"
options = (
    ('grpc.primary_user_agent', user_agent),
)
channel = grpc.insecure_channel(connect, options=options)

In Go, it looks like this:

endpoint = 'localhost:50041'
user_agent := "monkey 1.0.0"
var options []grpc.DialOption
options = append(options, grpc.WithInsecure())
options = append(options, grpc.WithUserAgent(user_agent)
grpc.Dial(endpoint, options)

I cannot find a way to do this with Tonic - am I missing this? Also, I unfortunately don't know anything about Hyper or Tower, so I don't know where to start implementing it.

[alce]

You can use an interceptor to set the user agent:

let channel = Channel::from_static("http://[::1]:50051").connect().await?;

let mut client = GreeterClient::with_interceptor(channel, |mut req: Request<()>| {
    req.metadata_mut().insert(
        "x-user-agent",
        MetadataValue::from_str("some-user-agent").unwrap(),
    );
    Ok(req)
});

[alertedsnake]

You can't set the metadata key user-agent as that's specifically marked as reserved, and setting a totally different key doesn't allow compatibility with other languages which use that key internally via the methods I described.

[alce]

You are right, this is a bug. user-agent header should be whitelisted. I'm not sure at this point if we can just remove it from the reserved headers list but will look into it. Thanks.